The present invention relates to a process for the isomerization of paraffin hydrocarbons catalysed by a mixture of an acidic ionic liquid catalyst and a metal salt.
Paraffin hydrocarbons with high degree of branching are known to be useful blending components for motor gasoline due to their high octane numbers. Such paraffin hydrocarbon fractions can be produced in an isomerization process increasing the octane number of the C4–C9 cuts. Isomerization of C4, C5 and C6 paraffins are common refinery processes based on use of a Friedel-Crafts catalyst such as AlCl3 or a group 8–10 metal (using current IUPAC nomenclature for the Periodic Table of elements with groups from 1–18) supported on a halogenated, preferably chlorinated, carrier. Processes including higher fractions (C7 to C9 hydrocarbons) meet with significant difficulties due to low selectivity and low octane number of the once-through products.
The use of combinations of aluminium halides (in particular AlCl3) and certain anhydrous metal chlorides or sulphates (in particular CuCl2 or CUSO4) for alkane isomerization is known and has been described in several scientific articles. Ono et al. (Chem. Lett. (1978), 1061; Chem. Lett. (1978), 625; J. Catal. 56 (1979) 47; J. Catal. 64 (1980) 13) describes pentane isomerization at room temperature to 323 K by a series of physical mixtures of AlCl3 or AlBr3 with approximately 30 different metal salts. The highest conversions were obtained for mixtures of AlCl3 with anhydrous copper (II) salts preferentially CuCl2 and CuSO4. It was noticed that maximum conversions occurred at defined was noticed that maximum conversions occurred at defined AlCl3/CuCl2 or AlCl3/CuSO4 molar ratios of 0.5. This suggested that a specific compound was formed as active species with super-acid property. This conclusion was supported by acidity measurements showing that the acid strength of the AlCl3/CuSO4 mixture was found to be relatively higher than that of AlCl3 alone. By independent synthesis (N. Kitayima, Y. Ono, J. Mol. Catal. 10 (1981) 121) the mixed metal chloride Cu(AlCl4)2 was obtained with 20 times higher isomerization activity at room temperature than physical mixtures of AlCl3 and CuCl2.
Another example on the use of combinations of AlCl3 based isomerization systems with metal salt additives is described in U.S. Pat. No. 5,202,519, which discloses mixtures of an aluminium halide (preferably AlCl3), calcium aluminate, a copper (II) salt (preferably CuCl2) and an alcohol. The molar ratio of aluminium halide:Cu salt was preferably 3:1 to 2:1. This mixture was shaped and the shaped particles were dried. This catalyst composition was used in the isomerization of C4–C10 alkanes and/or C5–C10 cycloalkanes at temperatures of up to 300° C., more preferentially at temperatures of 20–40° C. In the isomerization of n-pentane at 24° C., a conversion of 95.5% was obtained, however, the selectivity to isomerization products was lower than the selectivity to disproportionation and cracking products (C4, C6+). In the isomerization of methyl-heptane this catalyst composition showed a much lower conversion of 54.6% again with a low selectivity to isomerization products due to substantial conversion of the feed to higher and lower isoalkanes.
While known art using combinations of aluminium halides with (transition) metals concentrates on copper salts as most effective additives, the choice of additive is not restricted exclusively to this metal. In U.S. Pat. No. 5,358,919 alumina is impregnated with a sulphate solution of copper, iron, cobalt, nickel, manganese, zinc or magnesium. After a calcination step the obtained material was mixed with AlCl3 and a chlorinated hydrocarbon and this mixture heated to 40–90° C. This composite catalyst was used in isomerization reactions of alkanes and cycloalkanes preferably at temperatures of 20–50° C.
A different example on the use of metal salts as additives to a catalyst for paraffin isomerization is described in the article J. Chem. Soc., Perkin Trans 2, 1999, pages 2715–2718. In this case the hexane isomerization catalysed by a liquid superacid, trifluoromethanesulphonic acid (CF3SO3H) is investigated in the presence of FeCl3 and CuCl2. The experiments were carried out as batch reactions in a capped tube (containing hexane and CF3SO3H) without stirring in order to have a well defined contact area throughout the study of the isomerization kinetics. Adding a small amount (not specified) of FeCl3 and CuCl2, respectively, resulted in the activation of the system and a relative faster conversion of hexane to isomeric product was observed.
A relatively new class of acidic catalysts based on ionic liquids has been described in the literature (P. Wasserscheid, W. Keim, Angew. Chem., Int. Ed., 2000, V. 39, pages 3772–3789; T. Welton, Chem. Rev., 1999, V. 99, pages 2071–2083). This group of compounds also referred to as molten salts are constituted of:    (1) an inorganic anion, typically formed from metal halides, such as AlCl4−, Al2Cl7−, or other inorganic anions (SO42−, NO3−, PF6−, CF3SO3−, BF4− etc.), and    (2) an organic cation, typically derived from N-heterocyclic or alkylammonium entities.
The melting point of ionic liquids is relatively low and an increasing number of ionic liquids are described with melting points below room temperature. Below some characteristics of ionic liquids are listed:    (1) They have a liquid range of about 300° C.    (2) They are good solvents for a wide range of inorganic, organic and polymeric materials.    (3) They exhibit Broensted and Lewis acidity as well as superacidity.    (4) They have low or no vapour pressure.    (5) Most ionic liquids are thermally stable up to near 200° C., some ionic liquids are stable at much higher temperature (about 400–450° C.).    (6) They are relatively cheap and easy to prepare and upscale.    (7) They are non-flammable and easy in operation.    (8) They are highly polar but non-coordinating materials.
Ionic liquids most frequently demonstrate Lewis acidic properties once they are formed by metal halides. In many cases, however, the ionic liquids also show strong Broensted (proton) acidity. The proton acidity may originate both from the cation if it contains a proton at the quarternized N atom, or from the anion if it contains protons, for instance in HSO4−, H2PO4−.
Also HCl produced via partial hydrolysis for example of the chloroaluminate anion can explain strong proton acidity of the ionic liquids.
Lewis-acidic properties of ionic liquids are governed by two major factors: (1) the nature of the anion, and (2) the molar ratio of the organic part to the inorganic part (for instance, in the case of ionic liquids based on metal halides Me(Hal)n by the molar fraction of Me(Hal)n). If XMe(Hal)n<0.5, the ionic liquid is called basic; if XMe(Hal)n=0.5, this is the case of neutral ionic liquid, and finally if XMe(Hal)n>0.5, the ionic liquid can be classified as acidic or in some cases superacidic.
The effect of superacidity of ionic liquids is quite frequently observed for AlCl3-based compositions. Sometimes this effect is related to the presence of dry HCl in the system, which is dissolved in the ionic liquid. The Hammett function H0 for such systems (H0=−18) indicates superacidic properties of the ionic liquids comparable with those of HF—TaF5 (H0=−16) and “magic acid” HF—SbF5 or FSO3H—SbF5 (H0=−25). All these systems are much stronger acids as compared to the conventional 100% H2SO4 (H0=−12), which marks the border of superacidity. Such ionic liquids are also stronger than the solid superacids like SO4/ZrO2 (H0=−16), H3PW12O40 (H0=−13.5) or H-Nafion (H0=−12).
Room-temperature ionic liquids are promising media for a wide range of catalytic reactions including downstream oil processing, basic organic synthesis and fine chemicals production. Among these processes of potential commercial interest are various alkylation, oligomerisation and isomerization reactions.